1. Field of the Invention
This invention relates to a frequency demodulation circuit for demodulating a frequency-modulated (FM) signal.
2. Description of the Prior Art
In a magnetic recording and reproducing apparatus which perform low carrier FM recording and FM single side-band reproduction such as a commercial VTR, when the FM signal has a large frequency modulation index, a zero cross point often cannot be reproduced with fidelity in an FM demodulator, so that a signal inversion, or an inversion phenomenon is apt to occur. This phenomenon occurs when the level of lower side-band component J-.sub.1 becomes greater than the level of the FM fundamental wave component J.sub.0. Furthermore, when the reproduced noise is overlapped, the inversion phenomenon occurs due to the influence of the noise even when J-.sub.1 &lt;J.sub.0.
To avoid the occurrence of this inversion phenomenon, a frequency demodulator disclosed in U.S. Pat. No. 4,470,080 superposes pulses on an input FM signal before demodulating the FM signal. The superposed pulses occur at timings delayed by a specific time from the zero cross points of the fundamental wave component of the FM signal. The specific time is set such that the pulses are superposed on the peak points of the FM signal at the carrier frequency. This operation will be described in more detail with reference to the block diagram shown in FIG. 16 and the waveform diagram shown in FIG. 17.
The fundamental wave component i of an input FM signal h is output by a band-pass filter 20, and delayed by a prescribed time by a delay circuit 21 to form signal j. The signal j is passed through a limiter 22 to obtain a signal k, from which pulse signals l are obtained by a pulse generation circuit 23. The pulse signals l indicate the zero cross points of the delayed fundamental wave signal j, which is a condition essentially different from that of the present invention. Furthermore, by superposing the pulse signals l on the reproduced FM signal h by an adder 24, a signal m is obtained. Since a zero cross point exists at a point of a high modulation index, the inversion phenomenon does not occur.
However, the pulse signals l represent the time information of the zero cross points of the signal j of the fundamental wave which is delayed by the prescribed time, while the time information of the peak points of the input FM signal waveform is essentially different time information. Thus, the addition of signals having different time information causes the information of the FM signal to be distorted, so that the following problems are caused.
If it is assumed that an FM allocation of a VTR is 5 to 7 MHz, and the dark clip is 100, and the white clip is 200%, then the dark clip frequency becomes 3 MHz, and the white clip frequency becomes 9 MHz, and the range where the fundamental wave exists becomes 3 to 9 MHz. That is to say, the inversion period of the fundamental wave will change between a range of approximately 333/2 nsec and 111/2 nsec.
When the fundamental wave signal i is delayed by 100 nsec to form the signal j, the pulse-superposed FM signal m indicates that the phase relationship between the superposed pulses and the FM signal changes with frequency. That is, when the frequency of the fundamental wave signal i is low, the pulses are superposed on positions close to the peak points of the fundamental wave signal, but when the frequency of the fundamental wave signal is high, the pulses are superposed on positions shifted from the peak points of the fundamental wave signal. Therefore, the zero cross point cannot be restored at a point X.sub.1, so that what is called the black break is generated. At a point X.sub.2 where an excess zero cross is generated, what is called the white break is generated.
When the delay time is reduced to 50 nsec, the signals j, k, l, and m become signals j.sub.2, k.sub.2, l.sub.2, and m.sub.2, respectively. The signal m.sub.2 indicates that the white break is generated at the point X.sub.3. When the delay time is further reduced, the pulse signals will be superposed closer to the zero cross area, so that the waveform at the zero cross points of the reproduced FM signal is changed. This will detrimentally influence the frequency characteristics after demodulation. Therefore, mere reduction of the delay time is not desirable.
As described above in the conventional frequency demodulator as disclosed in U.S. Pat. No. 4,470,080, the prevention of the invention phenomenon cannot be achieved over the overall frequency range of the FM signal. Furthermore, the superposed pulses cause deterioration of the FM signal waveform.